1. Field of the Invention
The present invention relates to a method of laser-annealing an amorphous silicon film or a crystalline silicon film formed on an insulating substrate made of glass or the like to crystallize the film or to improve the crystallinity.
2. Description of the Related Art
In recent years, a technique has been widely researched in which laser-annealing is conducted on an amorphous silicon film or a crystalline silicon film (a silicon film having crystallinity such as polycrystal or microcrystal, which is not mono-crystal) formed on an insulating substrate made of glass or the like, that is, a non-monocrystal silicon film, to crystallize those films or to improve the crystallinity.
The crystalline silicon film formed by laser-annealing, for its high mobility, has been widely employed for a monolithic type liquid-crystal electro-optic device, etc., in which a thin-film transistor (TFT) is formed using the crystalline silicon film, and TFTs for pixel driving and drive circuits are then fabricated, for example, on a single glass substrate.
Also, a method has been preferred to employ in which pulsed laser beams such as an excimer laser is processed by an optical system so as to be formed into beams having a square spot of several cm2 in cross section or beams having a linear shape of several mm widthxc3x97several tens cm, and laser-annealing is conducted by scanning the laser beam thus processed (while the radiation position of the laser beams is moved relatively with respect to a surface to be irradiated), because it is improved in productivity and excellent industrially.
In particular, the use of the linear laser beam makes the productivity high because laser beams can be irradiated on the overall surface to be irradiated by the scanning operation conducted only in a direction perpendicular to the linear direction, which is different from a case in which spot-like laser beams that require the right and left scanning operation as well as the forward and backward scanning operation are used.
There arise several problems in conducting laser-annealing on the non-monocrystal silicon film by scanning spot-like or linear laser beams emitted from a pulsed laser beam source thereon.
For example, in the case where laser-annealing is conducted in the air, there arises such a problem that impurities of carbon contained in the air and other materials are liable to be mixedly inserted into the film, to thereby deteriorate the various characteristics such as the quality, the crystallinity or the mobility of the crystalline silicon film which has been annealed.
Also, in the case where laser-annealing is conducted by scanning beams which are spot-shaped or linear on the surface to be irradiated in a vacuum atmosphere or inactive gas atmosphere such as nitrogen, the following problems are caused in comparison with annealing in the air.
1) The crystallinity is deteriorated. That is, a high crystallinity cannot be obtained without largely increasing the energy density of a laser beam in comparison with annealing in the air.
2) The uniformity in the film of the crystal is deteriorated. Locations where the crystallinity is high and locations where the crystallinity is low are distributed in the film. For example, in the case where linear laser beams are scanned in a direction perpendicular to the linear direction of the beam, locations where the crystallinity is high and locations where the crystallinity is low appear in the form of a stripe pattern on the film surface. Accordingly, in the case where a plurality of thin-film transistors are manufactured using the fabricated crystalline silicon film, a variety of characteristics such as a threshold value or mobility are different depending on a position of the thin-film transistor on a substrate.
3) The use efficiency of an energy is deteriorated. For the purpose of enhancing the crystallinity, the energy density of laser must be increased. As the energy density is increased, the power consumption is also increased. In addition, the entire laser irradiating device including a laser oscillator and a circuit, a gas and an optical systems is largely consumed, resulting in the increased costs of a manufactured device. Also, although the crystallinity is increased as the energy density of laser is increased, an entire film which has been subjected to laser-annealing is remarkably roughened, thereby making it hard to manufacture the device by processing the film.
The present invention has been made to solve the above problems of the conventional device, and therefore an object of the present invention is to provide a laser-annealing method which is capable of remarkably improving crystallinity and uniformity, as well as the use efficiency of energy.
In order to solve the above problems, according to one aspect of the present invention, there is provided a laser-annealing method which comprises the steps of: a first step of cleaning a non-monocrystal silicon film formed on a substrate; and a second step of laser-annealing said non-monocrystal silicon film in an atmosphere containing oxygen therein; wherein said first and second steps are conducted continuously without being exposed to the air.
In the above method, it is preferable that said second step is conducted after an upper surface of said non-monocrystal silicon film has been oxidized in the atmosphere containing oxygen therein.
According to another aspect of the present invention, there is provided a laser-annealing method which comprises the steps of: a first step of cleaning a non-monocrystal silicon film formed on a substrate; a second step of oxidizing an upper surface of said non-monocrystal silicon film to form a silicon oxide film; and a third step of laser-annealing said non-monocrystal silicon film; wherein at least said first and second steps of the respective steps are conducted continuously without being exposed to the air.
In the above method, it is preferable that said third step is conducted in a nitrogen atmosphere.
According to still another aspect of the present invention, there is provided a laser-annealing device which comprises at least a cleaning chamber and a laser irradiation chamber, in which a substrate to be processed is transported between said cleaning chamber and said laser irradiation chamber without being exposed to the air.
According to yet still another aspect of the present invention, there is provided a laser-annealing device which comprises at least a cleaning chamber, a preliminary heating chamber and a laser irradiation chamber, in which a substrate to be processed is transported between said cleaning chamber and said preliminary heating chamber without being exposed to the air.
In this specification, the above term xe2x80x9ccontinuouslyxe2x80x9d means that no step in which impurities or other undesired materials are stuck on the non-monocrystal silicon film exists between said first and second steps.
Accordingly, for example, to provide a substrate transporting step, an alignment step, an annealing step, a step of heating the substrate up to a temperature necessary for the second step, a step of dehydrogenation step by heating, and so on fall within the xe2x80x9ccontinuationxe2x80x9d in this specification.
On the other hand, in the case where a step of exposing a non-monocrystal silicon film to a specific atmosphere that changes the quality of the film, a ion doping step, a film forming step, an etching step, a plasma processing step, a film coating step, and so on are conducted between the above first and second steps, these steps do not fall the definition of the xe2x80x9ccontinuationxe2x80x9d in this specification.
According to the present invention, in laser-annealing the non-monocrystal silicon film to crystallize the film or improve the crystallinity, the upper surface of the non-monocrystal silicon film is oxidized in an atmosphere containing oxygen therein, to particularly form a silicon oxide film 100 xc3x85 or less in thickness, and thereafter a laser beam is irradiated onto the silicon oxide film.
Also, according to the present invention, a laser beam is irradiated onto the non-monocrystal silicon film in a state where it is disposed in an atmosphere containing oxygen therein.
Further, according to the present invention, a step of cleaning the non-monocrystal silicon film to remove a natural oxide film or impurities, and a step of irradiating a laser beam onto the non-monocrystal silicon film in an atmosphere containing oxygen therein or a step of forming a silicon oxide film on the upper surface of the non-monocrystal silicon film in an atmosphere containing oxygen therein are continuously conducted without being exposed to the air.
When the upper surface of the non-monocrystal silicon film is oxidized to form the silicon oxide film 100 xc3x85 or less in thickness, and the silicon oxide film is laser-annealed in this state, not only the crystalline silicon film purer than that formed by laser-annealing in the air is obtained, but also the following excellent characteristics are obtained in comparison with a case of conducting laser annealing in the air or in another atmosphere.
1) The crystallinity of the crystalline silicon film is improved.
2) The crystallinity of the crystalline silicon film is uniformed in the film surface.
3) The energy density of laser necessary for crystallization is lowered.
With the formation of the very-thin oxide silicon film, it is presumed that the reflection/discharge of energy of a laser beam irradiated onto the non-monocrystal silicon film is suppressed so that a given energy is kept within the film. Therefore, a large amount of energy can be given to the non-monocrystal silicon film more than a case where no silicon oxide film is provided, to thereby improve the crystallinity.
Simultaneously, because the nonuniformity and dispersion of the energy density for each pulse of a laser beam are prevented, the crystallinity is also uniformed in quality within the film surface.
Further, because the reflection/discharge of a laser energy to an atmosphere is reduced so that the energy is effectively used for crystallization, the energy density of an irradiated laser beam can be lowered.
When a laser beam is irradiated to a non-monocrystal silicon film in a state where silicon oxide film is formed on the non-monocrystal silicon film at a high energy density as in the state where no silicon oxide film is provided, an excessive energy is given though an energy loss is reduced, with the result that the entire film is remarkably roughened although the crystallinity is enhanced. It is very hard to manufacture a device such as a thin-film transistor using such a film.
Also, it is preferable that the upper surface of the non-monocrystal silicon film is cleaned by HF aqueous solution or aqueous solution containing HF and H2O2 therein to remove the natural oxide film before conducting laser-annealing. It is preferable that the subsequent step of manufacturing the silicon oxide film, or step of laser-annealing in an atmosphere containing oxygen therein is conducted while heating the substrate because the rate of forming the silicon oxide film is improved. The above steps may be conducted while irradiating ultraviolet rays on the film.
In particular, it is preferable that the above cleaning step and the subsequent laser annealing step conducted in an oxygen atmosphere are continuously conducted without being exposed to the air, or the cleaning step, a heating (silicon oxide film forming) step which is conducted in an oxygen atmosphere, and the laser-annealing step are continuously conducted without being exposed to the air.
With the above method, the silicon oxide film is formed from the very clean upper surface of the non-monocrystal silicon film. As a result, the silicon oxide film formed becomes more uniform in thickness and quality, to thereby improve the uniformity of the quality of the film crystallized by laser-annealing within the substrate surface.
Further, the invasion by impurities into the non-monocrystal silicon film during laser-annealing is more reduced. As a result, a variety of characteristics such as the mobility or the threshold value of the device such as a thin-film transistor which is manufactured using the above film can be more stabilized within the substrate surface as well as between lots.
The atmosphere containing oxygen is preferably comprised of only oxygen, or the mixture gas of oxygen and an inactive gas such as nitrogen, helium or argon. The mixture gas preferably contains oxygen of 1% or more, more preferably 5% or more under the atmospheric pressure. When the oxygen content is 1% or less, a period of time necessary for forming a sufficient silicon oxide film becomes extremely long, or the silicon oxide film cannot be formed, thus obtaining the insufficient effect of the present invention. Therefore, it is not for practical use. If the oxygen content is 5% or more, the effect of the present invention is stably obtained.
In the case where air is used as an atmosphere containing oxygen in forming the silicon oxide film, the impurities such as carbon in the air is mixed into the film to be annealed, often resulting in cases in which the mobility of the crystalline silicon film and other various characteristics are lowered, or the characteristics for each lot are unstabilized. It should be noted that it is effective that, after the silicon oxide film is manufactured in another atmosphere, the film is laser-annealed in the air atmosphere.
Also, it is particularly preferable that oxygen or inactive gas which constitutes an atmosphere containing oxygen is 99.9% (3N) or higher but 99.99999% (7N) or lower in purity. With an atmosphere using the gas of this purity, carbon, water, hydrocarbon and other impurities are prevented from being mixed into the crystalline silicon film, to thereby obtain the crystalline silicon film which is stabilized in quality and characteristics and is excellent in characteristic. When the purity of oxygen or inactive gas which forms an atmosphere is less than 3N, there is little difference from a case of using the air atmosphere, and the film characteristic is liable to be unstabilized by the impurities. Also, even though the gas having a high purity more than 7N is used, there is no large difference from a case where the purity is 7N or less, but the costs are increased. Therefore, it is not preferable.
Moreover, when the thickness of the above silicon oxide film is set to 100 xc3x85 or more, the amount of mixture of the silicon oxide film into the crystalline silicon film is increased by the irradiation of a laser beam, to thereby lower a variety of characteristics such as the crystallinity or the mobility of the crystalline silicon film. On the other hand, when the thickness of the silicon oxide film is too narrowed to about 5 xc3x85 or less, the above-mentioned effects of the present invention is remarkably deteriorated. The thickness of the silicon oxide film is suitably set to 5 to 100 xc3x85, preferably 10 to 50 xc3x85, and more preferably 20 to 40 xc3x85.
The pressure applied when conducting laser-annealing may be atmospheric pressure. In the case where the pressure applied when conducting laser-annealing is reduced to atmospheric pressure or less, in particular, to 0.01 to 700 Torr, the upper surface or the entirety of the crystalline silicon film is less roughened by the irradiation of pulsed laser beams by plural times, which is preferable. In other words, the pulsed laser beam irradiation resistance of the crystalline silicon film is improved so that a film less roughened is obtained. In the case where the pressure applied when conducting laser-annealing is more than 700 Torr, the roughness of the film is nearly identical with that in the case of atmospheric pressure. In the case where the pressure is less than 0.01 Torr, such effects as improvements in crystallinity, uniformity in quality, and the energy efficiency are then remarkably deteriorated.
It is preferable that the irradiation of a laser beam is conducted by scanning a laser beam which is slot-shaped or linear in cross section on a surface to be irradiated.
Also, it is preferable that the laser beam is irradiated from a pulsed laser source.
To implement the present invention, the non-monocrystal silicon film is exposed to an atmosphere containing oxygen therein, or the upper surface of the non-monocrystal silicon film is oxidized by heating or the irradiation of ultraviolet rays under the condition where the non-monocrystal silicon film is exposed to an atmosphere containing oxygen therein, and then the film thus obtained is laser-annealed.
After the upper surface of the non-monocrystal silicon film has been oxidized in the atmosphere containing oxygen in one chamber, the film may be laser-annealed in the atmosphere containing oxygen or in another atmosphere in another chamber.
Also, in the case where laser-annealing is conducted in an atmosphere containing oxygen therein within an atmosphere-controllable vessel, oxidation and laser-annealing can be conducted within one vessel, thus reducing the manufacture processes. In this case, it is preferable that a substrate is heated when conducting laser-annealing.
The silicon oxide film according to the present invention is completely different from a cap layer (what prevents the roughness (ridges) on the surface of the silicon film which is caused during laser-annealing by the mechanical strength of a film which is formed as a silicon oxide film or a silicon nitride film several 1000 xc3x85 in thickness on the amorphous silicon film when conducting laser-annealing mainly using a small-output continuous oscillation laser).
The above thick silicon oxide film, when using a large-output pulse laser such as an excimer layer as in the present invention, allows a large amount of silicon oxide to be mixed into a silicon film when conducting laser-annealing as described above, resulting in the deterioration of the quality and the characteristics of the crystalline silicon film formed.
When laser-annealing is conducted in the state where the cap layer is provided, the film is crystallized in the state where it is pressed against the cap layer. As a result, the growth of crystal is suppressed to deteriorate the crystallinity of the crystalline silicon film formed. In addition, a large stress remains inside the crystalline silicon film.
In the present invention, because the silicon oxide film is extremely thin, the growth of crystal is hardly suppressed, with the results that a higher crystallinity is obtained than that of the cap layer, and an internal stress can be also considerably reduced.
Hence, a thickness of the film that produces a mechanical strength to the extent that the ridges can be suppressed is improper for the silicon oxide film according to the present invention. Because the silicon oxide film according to the present invention is extremely thin to 100 xc3x85 or less, most of the silicon oxide film is scattered by the plural times of irradiation of pulsed laser beams so as to be removed.
The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.